How about an original enemy? Like say the destruction of Vulcan caused a Vulcan living on another planet to lose his sh*t and go batsh*t crazy, or someone who wasn't bad in the original universe goes bad in this one.

Downey Jr is the best Tony Stark ever. I bet what the studio will do when they recast is that they'll try looking for an actor that can best emulate Downey Jr. because they'll be afraid of casting a new tony stark but with a completely different personality....we'll just have to wait and see.